Method of drying a porous body

ABSTRACT

A method of making a dried porous ceramic body having interconnected surface pores and interior channels. The body is formed by extrusion from a paste and is irradiated with microwave radiation to heat it for an initial period at a rate of at least 15° C./minute, after which the irradiation is reduced so that the body is heated at a rate of at most 1° C./minute. Heat loss from the external surface of the body, during heating, is resisted.

This Invention relates to a method of drying a porous body.

According to one aspect of the invention there is provided a method ofdrying a porous body having an external surface provided with pores incommunication with interconnected pores or channels in its interiorcontaining a liquid, the method comprising the steps of:

subjecting the body to heat energy input by irradiating it withelectromagnetic radiation for an initial period to cause the body toundergo an average rate of temperature increase over the initial periodof at least 2° C./minute; and

reducing the rate of heat energy input or power of the irradiation for asubsequent period to reduce the rate of temperature increase of the bodyto at most 1 ° C./minute.

The ratio of the initial period: subsequent period may be 1:10-10:1,preferably 1:4-4:1.

According to another aspect of the invention there is provided a methodof drying a porous body having an external surface provided with poresin communication with interconnected pores or channels in its interiorcontaining a liquid, the method comprising the steps of:

subjecting the body to heat energy input by irradiating it withelectromagnetic radiation to cause the body to undergo an average rateof temperature increase of at least 2° C./minute; and

resisting heat energy loss from the external surface of the body.

Preferably the method of the present invention comprises both said stepof reducing the rate of heat energy input, and said step of resistingheat energy loss. In this case there may be a ratio between the initialperiod and the subsequent period of 1:10-10:1, the method includingresisting heat energy loss from the external surface of the body.

By drying is meant a reduction in the liquid content of the body to adesired value. Preferably, the drying is carried out to a degree wherethe body can be handled without causing any damage thereto orunacceptable deformation thereof. For bodies comprising consolidatedparticulate material of the type described hereunder, a dried bodyhaving lost about 2-5% of its wet mass during drying has been found tobe sufficiently dry for handling purposes.

Resisting said heat energy loss may be by thermally insulating the body,eg by providing thermal insulation in contact with or in close proximityto the external surface of the body. The heat energy loss may, insteador in addition, be resisted by supplying heat to the external surface ofthe body from an external heat source, eg by heating walls defining acavity around the body and/or by maintaining an ambient atmospheresurrounding the body at a predetermined temperature, eg by increasingthe temperature of the ambient atmosphere around the body, and/or thetemperature of said walls, during said temperature increase of the body.In particular, resisting heat energy loss from the exterior surface ofthe body may comprise heating the external surface of the body by meansof radiant heat, radiated on to the external surface of the body from atleast one radiant heat source, such as a radiant heating element.Preferably, the temperature of said walls, and/or the temperature ofsaid ambient atmosphere, are kept as close as feasible or practicable tothe temperature of the external surface of the body. In particular,resisting heat energy loss from the external surface of the body may besuch as to keep the external surface of the body at substantially thesame temperature as the interior of the body, so that no unacceptabletemperature gradient exists in the body between any part of its interiorwhich is hotter than the external surface, and the external surface,routine experimentation being employed to determine the reduction ofheat loss required. As indicated above, the method may comprise heatingthe environment surrounding the body to cause its temperature toincrease progressively as the temperature of the body increases, theenvironment being defined by a cavity having a wall or walls directed atthe porous body and by the atmosphere in the cavity around the body, andthe temperature difference between the external surface of the porousbody and the wall surface temperature of the cavity preferably beingrestricted to an acceptably small value. If desired the surface of thethermally insulating material which faces the body may be heatreflective.

The present invention accordingly has, as a feature thereof, subjectingthe body to electromagnetic radiation, to cause the body to undergo atemperature increase, with as small a temperature gradient as feasibleor practicable between its interior and its said external surface.

The body will usually be cylindrical or spherical. Typically, the bodywill comprise a consolidated particulate material, eg a ceramicmaterial, such as α-alumina. Naturally, the body may comprise any otherporous material, such as wood. By drying, as contrasted with egcalcining or sintering, is meant that the heating is to a temperatureand for a period in an environment such that, if the body comprisesinorganic material, the body undergoes no chemical changes to theinorganic material, changes to the body being confined to reversiblephysical changes related to the reduction of the liquid content thereof.Naturally, if the body comprises organic material, eg wood, certainirreversible chemical changes may occur in the material in addition tothe physical reduction of the liquid content thereof.

When the body is formed from consolidated particulate material, theliquid contained in the body will typically be water, the water beingpresent in free and/or bound form in the body. Before drying, the bodymay have a moisture content of 14-16% by mass, typically 14.5-15.5% bymass. After the drying, due to loss of moisture, the wet mass of thebody may have been reduced by at least 2% by mass, typically 4% by mass.Accordingly, the method may include the step of consolidating aparticulate material to form the body with a moisture content, beforedrying, of 14-16% by mass, the drying acting to reduce the moisturecontent thereof by 2-5% by mass, to a value of 9-14% by mass. Moreparticularly, consolidating the particulate material may be byextrusion, the initial period being measured, for any part of the body,from the moment of extrusion of that part, and the extrusion being intoa cavity having a wall or walls spaced at most 100 mm from the body, thetemperature of the external surface of the body and the wall surfacetemperature of the cavity increasing simultaneously and progressively inthe direction of extrusion of the body during the initial period,preferably so that any part of the body is opposed to a part of thecavity wall or walls which is at substantially the same temperature. Themethod may include causing or allowing moisture expelled as vapour fromthe porous body by the drying to issue from the cavity, so that thepressure in the cavity remains substantially constant during the drying.

In a preferred embodiment, the body is subjected to microwave radiationhaving a frequency of 0.3×10⁹ -1×10¹⁰ Hz, usually 1-10 GHz, eg 2.45 GHzbeing delivered to the body at a power of 2-4 kW/kg, eg 3 kW/kg, of wetbody mass. When using microwave radiation, a small temperature profilein the body of the type described above can be promoted by arranging themicrowave radiation to provide, in the body, a flux density of microwaveradiation, which is as constant as is feasible or practicable.

The initial period may have a value of 0.2-20 minutes, preferably 0.5-5minutes, eg 1 minute.

In a particular embodiment of the method, the electromagnetic radiationmay be microwave radiation having a frequency of of 0.3×10⁹ -1×10¹⁰ Hz,the initial period having a duration of 0.2-20 minutes and the averagerate of temperature increase over the initial period being at least 15°C./minute, the microwave radiation being delivered to the body at apower of 2-4 kW during the initial period and at a power of at most0.5-2 kW during the subsequent period.

The average rate of temperature increase during the initial period ispreferably at least 15° C./minute, eg 30° C./minute.

The maximum temperature to which the body is heated may be at most thattemperature, depending on the composition of the body, at which noundesirable effects occur in the body, such as cracking, blistering orthe like.

The rate of heat energy input to the body may reduce progressively on acontinuous basis or conveniently stepwise, having a high value for theinitial period and reducing by eg one or more steps to a lower value orvalues, for the subsequent period. It should be noted that the rate oftemperature increase during the subsequent period can in principle bezero and/or negative, so that there is a temperature plateau and/or atemperature decrease or cooling of the body, during the subsequentperiod.

As mentioned above, resisting the heat loss from the body may be byproviding thermal insulation around the body. Preferably, the insulationis located in close proximity to the external surface of the body. Theinsulation may be spaced at most 50 cm from the external surface of thebody, typically at most 10 cm from the external surface of the body, eg2 cm from the external surface of the body. Instead of or in addition tothe insulation, the heat loss from the body may be resisted by causingthe ambient environment or atmosphere surrounding the body to be at thesame temperature as the external surface of the body, eg by heating thebody in a heated cavity, the cavity having a vapour outlet to allowescape of vapour from liquid evaporated from the interior of the body.In other words, the ambient temperature around the body may bemaintained at substantially the same temperature as the external surfaceof the body, eg by heating the inner wall or walls of a cavitysurrounding the body at the same rate as the rate of temperatureincrease of the body.

Subjecting the body to the electromagnetic or microwave radiation may,as indicated above, be carried out with the body in a microwave cavity.Naturally, the size and shape of the cavity may be selected depending onthe size and shape of the body to be dried. As mentioned above, insteadof or in addition to having the wall or walls of the cavity heated tocorrespond in temperature with the body, the wall or walls may beprovided with thermal insulation. The thermal insulation may be providedaround the body in the cavity, being eg of silica/alumina insulatingmaterial. Depending on the degree of thermal insulation and the size ofany space around the body between the body and the wall or walls of thecavity, which during heating will be filled with vapour derived from theliquid in the body, it may be possible to dispense with separate heatingof the cavity to keep it at the same temperature as the body, the cavitywall or walls being heated by convection and/or conduction from thevapour, and by radiation from the body, so that the temperaturedifference between the external surface of the body and the cavity wallsurface is acceptably small.

The cavity is thus preferably shaped or arranged to fit snugly aroundthe body so that a relatively small space is present between the bodyand walls defining the cavity, the small space facilitating themaintenance of a high vapour concentration between the cavity and thebody, to promote maintenance of the same temperature at the cavity wallsurface and at the external surface of the body.

The cavity may be provided with at least one electromagnetic radiationsource, such as a microwave source. The microwave source, eg amagnetron, may be adjustable as regards its power output. In a preferredembodiment, there are several such sources, which may be arranged and/oroperated to provide a multimode or tuned cavity.

The microwave sources may be arranged and/or spaced from each other inthe cavity in which the body is located, in order to promote a constantflux density throughout the body, in turn to promote a constanttemperature throughout the body. Routine experimentation will benecessary to determine the optimum microwave intensity, wavelength andpositioning of the magnetrons, and the period or periods required fordrying, depending on the composition of the body. Microwaves ofcommercially available wavelength, such as 2.45 GHz, will usually beused.

The reduced rate of temperature increase during the subsequent periodmay be achieved by subjecting the body to further electromagneticradiation (eg microwave radiation) at a reduced average heat energyinput; or by subjecting the body to any other suitable conventional(radiative, convectional and/or conductive) means of heating.Conveniently, during the subsequent period, the heat energy input issuch that the body is maintained at a substantially constant temperatureor the body is subjected to a substantially reduced rate of temperatureincrease, so that its temperature increases slowly, if at all.

The body dried in accordance with the invention may in particular be aceramic filtration support suitable for use in supporting a filtrationmembrane in a filter element used for micro- or ultrafiltrationapplications, such as the supports described in the Applicant'sco-pending patent application No. 08/983,080. It will be appreciated,however, that the method in accordance with the invention is not limitedto the drying of ceramic filtration supports for filter elements, butextends also in particular to the drying of other extruded or shapedceramic bodies, such as clay bricks, clay tiles or the like, and indeedalso to shaped bodies formed from particulate solids which are notnecessarily ceramics. The method may also be applied to non-extrudedporous bodies containing liquid, eg bodies comprising elastic porousmatrices such as wood, as well as to bodies comprising non-elasticporous matrices such as porous mineral or moulded ceramic bodies.

The invention extends to a process for forming a dried body of porousmaterial which comprises the steps of:

consolidating particulate material containing a liquid to form a porousbody having interconnected pores in its interior in which the liquid iscontained; and

drying the body, the drying being as described above.

The consolidating may be by extrusion or moulding, or by other suitableshaping methods.

Naturally, the particle size and/or particle size distribution of thematerial forming the body will be selected to provide a body or productof the desired percentage porosity and pore size distribution.

In accordance with the method of the present invention the externalsurface of the body may be kept as close as possible to the temperatureprevailing in its interior, by the employment of one or more of thefollowing optional features:

surrounding the body with thermally insulating material, the insulatingmaterial being more or less closely spaced from the external surface ofthe body;

surrounding the body with an inwardly directed heat reflecting surface,eg on the insulating material;

surrounding the body with heated cavity walls to compensate forradiative heat loss from the external surface of the body; and/or

introducing a heated fluid such as heated air to any space surroundingthe element to compensate for any heat loss from the external surface ofthe body arising from convective or conductional heat transfer.

Naturally, two or more, or all, the above features can be employedsimultaneously,

The invention will now be described, by way of example, with referenceto the following worked Example, and with reference to the accompanyingdiagrammatic drawings, in which:

FIG. 1 shows a schematic sectional side elevation of an installation fordrying a porous body in accordance with the method of the presentinvention;

FIG. 2 shows a schematic sectional side elevation of a variation of theinstallation of FIG. 1;

FIG. 3 shows a schematic sectional side elevation of another variationof an installation for drying a porous body in accordance with themethod of the present invention; and

FIG. 4 shows a schematic sectional side elevation of a further variationof the installation of FIG. 3.

EXAMPLE

The feasibility of the method of the invention has been demonstrated forthe drying of elongated ceramic filter element profiles of the typedescribed in the applicant's co-pending patent application No.08/983,080. To make such filter elements, a green paste mixture isformulated prior to drying. The exact composition of the green pastemixture is set out in the following Table, Table 1, after which theprocess of preparation of the green paste is described in detail.

                  TABLE 1                                                         ______________________________________                                        % (Wet                                                                          basis) Compound Code Function                                               ______________________________________                                        70.4   Alcoa Tabular  A       Basic powder                                       Alumina T-60-325                                                              STD (99% < 45 μm                                                          3.7 Alcoa Reactive B Basic powder                                              Calcined Alumina:                                                             A17 NE (90% < 8 μm)                                                       5.4 α-Alumina C Plasticizer/sintering                                    monohydrate  aid                                                              powder (supplied                                                              under the name or                                                             code KCM GC                                                                   Powder -- obtainable                                                          from Keith                                                                    Ceramics)                                                                    2.7 Methyl cellulose D Plasticizer/binder                                      (supplied under the                                                           name or code                                                                  Celocol HPM                                                                   15000 DS --                                                                   obtainable from                                                               Courtaulds                                                                    Chemicals)                                                                   2.0 Sletwyn Kaolin E Flux material                                             (supplied by                                                                  Rainbow Industrial                                                            Chemicals)                                                                   0.9 Polyalkylene Glycol F Lubricant                                            (supplied under the                                                           name or code                                                                  Breox 75W --                                                                  18000 -- obtainable                                                           from British                                                                  Petroleum)                                                                   14.9 Water                                                                    Σ100.0                                                                ______________________________________                                    

The various constituents of the composition in the Table 1 have beengiven alphabetic codes for ease of explanation of the paste/extrusionbody preparation process in the following Example.

A paste/extrusion body was prepared by the cooling of the constituentsA, B, C, D, E, and a solution of F in 43% of the total mass of water, toa temperature of 10° C. Constituents A, B, C, D and E were then mixedtogether in the following sequence of steps. Constituent A was mixedwith constituent B for 5 minutes in a ribbon blade mixer. Constituent Cwas then added to the mixture and mixing was carried out for a further 5minutes. Constituent D was then added to the mixture and mixing wascarried out for a further 5 minutes. Constituent E was then added to themixture and mixing was carried out for a further 15 minutes. 41 % of thetotal mass of the water was then added to the mixture and mixing wascontinued for a further 30 minutes. The moist powder mixture obtainedwas then stored in a sealed container overnight at a temperature ofabout 10° C. Constituent F was then added to the moist powder andfurther mixing was carried out in a high shear mixer for 10 minutes. Theresultant paste/extrusion body mixture was then stored and aged in asealed container at a temperature of 10° C. for 3 days. After additionof the last 16% of the total mass of the water, the aged paste was mixedin a high shear mixer for 90 seconds immediately prior to extrusionthereof.

The die was sized to extrude a profile of circular cross-section ofabout 98 mm diameter, having a plurality of filtration passagesamounting to thirty-six in number, each of 9 mm diameter, which extendthe length of the profile, parallel to one another and to the profile.The die also provided the profile with a central drainage passage alongits length, coaxial and parallel with the profile and of 9 mm diameter.The passages were arranged in three circular rows, and the rows wereequally radially spaced from each other and from the axis and surface ofthe profile. They formed three concentric circles when seen in endelevation, namely an external circle of eighteen passages and an innercircle of six passages, and an intermediate circle of twelve passages.The profile is intended to form a support for a filter membrane.

In FIG. 1, reference numeral 10 generally designates an installation forcarrying out the method of the present invention. In the drawing,reference numeral 12 designates a hollow multimode microwave cavity,within which is located a porous body in the form of the filter elementwhose formulation and extrusion are described above, the element beingdesignated 14. The element 14 is shown standing upright on one endthereof, on a slab 16 of insulating material. The element 14 is shownfurther surrounded by a cylinder 18 of the same insulating material,which rests, upright, on the slab 16 concentric with the element 14. Thecavity 12 is surrounded by a plurality of circumferentially spacedmagnetron microwave radiation applicators 20 arranged to directmicrowaves into the cavity 12 at a frequency of 2.5 GHz.

The insulating material comprises a consolidated mixture of 80% by massparticulate Al₂ O₃ and 20% by mass particulate SiO₂. The inner surfacesof the walls, roof and floor of the cavity 12 are coated with a heatreflective material, as are the inner surface of the cylinder 18 and theupper surface of the slab 16.

The slab 16 and cylinder 18 respectively in fact form linings for thefloor and side wall of the cavity, and there is a radial spacing ofabout 9 mm between the element 14 and the cylinder 18. The ratio betweenthe volume of this radial space and the volume of the element 14(including the volume of its internal filtration passages) is about1:0.026.

The microwave applicators 20 are tunable as regards their power output,having, in total, a maximum (100%) power output of about 6 kW.

Turning to FIG. 2, the same reference numerals designate the same partsindicated in FIG. 1, unless otherwise specified. The main differencebetween the installation 10 in FIG. 1 and that of FIG. 2, is that theinstallation 10 is shown having an additional inner cylinder 22 of thesame insulating material as the cylinder 18 surrounding the element 14.Furthermore, the cavity 12 is defined by metal walls 24 and base 26, thecavity 12 also being surrounded by the radiation applicators (notshown).

Turning to FIGS. 3 and 4, the same reference numerals indicate the sameparts as in FIGS. 1 and 2 unless otherwise specified. In FIG. 3, theelement 14 is shown standing upright on one end thereof, on the slab 16of insulating material, with no insulating material surrounding theelement 14. The installation 10 of FIG. 4 is similar to that of FIG. 3,except that in FIG. 4 the additional cylinder 22 of insulating materialsurrounds the element 14.

Two tests, Test 1 and Test 2, were carried out to evaluate the method ofthe invention, in terms of which two filter elements 14 respectivelymade as described above were dried in the installation of FIG. 1. Test 1was in accordance with the invention, and Test 2 was a control, not inaccordance with the invention. Shore hardness was measured in accordancewith DIN 53 505 (ISO P868) as a percentage, before drying and at variousstages during drying.

During the drying the power output of the microwave applicators 20 wasvaried from time to time, and various temperatures were periodicallymonitored, inside the channels of the filter elements, at the externalwalls of the filter elements, and in the interiors of the elements,adjacent their external walls.

The masses of the profiles were also monitored from time to time, todetermine the degree of drying. Details are set forth in the followingTable, Table 2.

                  TABLE 2                                                         ______________________________________                                        Test Time   Power   Mass   Hardness                                                                              T.sub.w                                                                            T.sub.c                                                                             T.sub.s                           No (min) % (g) Shore % (° C.) (° C.) (° C.)            ______________________________________                                        1    0      --      1987.6 24      24   24    24                                 (start)                                                                       1 100                                                                         3 50 1908.5 90 64 90 90                                                      2 0 -- 1955.1 24 24 24 24                                                      (start)                                                                       3 25  1947.35 50 56 67 54                                                     8 25 1873.5 87 87 90 74                                                       28  25 1663.5 99 99 150  78                                                ______________________________________                                    

In Table 2, T_(w) is the external surface temperature of the element inquestion, T_(c) is the temperature in a channel thereof, and T_(s) isthe temperature below its surface, adjacent its external surface. ForTest 1, values were measured before the start of heating, heating wasstarted at 100% power, with regard to microwave applicator output, andcontinued for 1 minute, after which it was reduced to 50% power andcontinued for a further 2 minutes. Heating stopped after 3 minutes,after which various values shown in the Table 2 were again measured. Inthe case of Test 2, values were measured initially, and respectivelyafter 3 minutes, 8 minutes and 28 minutes, heating taking place at 25%power, with regard to the microwave applicator output.

The Tests showed that Test 1, using a high initial drying power,followed by a low power, resulted after 3 minutes in a stable andsubstantially dry and hard filter element which could be furtherprocessed, eg by calcining and sintering. In case of the control, Test2, the same results could ultimately be obtained, but a considerablylonger drying period was required, longer by about an order ofmagnitude.

Further tests were carried out to evaluate the method of the inventionin terms of which filter elements, respectively made as described above,were dried in the installations of FIGS. 3 and 4. Tests 3 and 4 were inaccordance with the invention using the installation of FIG. 3, andtests 5 and 6 were in accordance with the invention using theinstallation of FIG. 4. The ambient temperature and relative humidityoutside the cavity were respectively 21-22° C. and 57-58%. The resultsof the tests are set forth in the following tables, Table 3 and Table 4below.

                  TABLE 3                                                         ______________________________________                                        Test Time   Power   Mass   Hardness                                                                             T.sub.w                                                                            T.sub.c                                                                              T.sub.s                           No (min) % (g) Shore % wall channel solid                                   ______________________________________                                        3    0      --      2014.5 ˜24                                             1 100                                                                         2 50                                                                          3 50 1921.3 85   91                                                              80                                                                            85                                                                         6.5 --                                                                        13 --                                                                         21 --                                                                         26 --                                                                        4 0 -- 1889.6 ˜24                                                        1 100                                                                         2 50                                                                          3 50 1785.8 85   92                                                              80                                                                            85                                                                         6.5 --                                                                        13 --                                                                         21 --                                                                         26 --                                                                      ______________________________________                                    

In the case of Tests 3 and 4, heating was started at 100% power, withregard to microwave applicator output, and continued for 1 minute, afterwhich it was reduced to 50% power and continued for a further 2 minutes.Heating stopped after 3 minutes, after which a value shown in Table 3for Test 3 was again measured. Tests 3 and 4 resulted in blistersforming on the filter element and in Test 3 a cracking sound was heardat the one minute period during heating. For Tests 3 and 4, a moistureloss of 4.63% by mass and 5.49% by mass respectively was recorded.

                  TABLE 4                                                         ______________________________________                                        Test Time   Power   Mass   Hardness                                                                             T.sub.w                                                                            T.sub.c                                                                              T.sub.s                           No (min) % (g) Shore % wall channel solid                                   ______________________________________                                        5    0      --      1890.9 ˜24                                             1 100                                                                         2 50                                                                          3 50 1814.6 95   91                                                              85                                                                            95                                                                         6.5 --                                                                        13 --                                                                         21 --                                                                         26 --                                                                        6 0 -- 1890.1 ˜24                                                        1 100                                                                         2 50                                                                          3 50 1787.5 90   92                                                              80                                                                            90                                                                         6.5 --                                                                        13 --                                                                         21 --                                                                         26 --                                                                      ______________________________________                                    

In the case of Tests 5 and 6, certain values were measured before thestart of heating, heating was started at 100% power, with regard tomicrowave applicator output, and continued for 1 minute, after which itwas reduced to 50% power and continued for a further 2 minutes. Heatingstopped after 3 minutes. Tests showed that a substantially dry and hardfilter element which could be further processed was obtained with noblisters forming on the external surface of the filter element. ForTests 5 and 6, a moisture loss of 4.04% by mass and 5.43% by massrespectively was recorded.

The method of the invention provides a method of drying, suitable fordrying green ceramic bodies, which is particularly suitable for thedrying of ceramic filter elements in the green state, after extrusionthereof. Short drying times are possible, compatible with extrusionrates of such filter elements, which can be typically extruded at a rateof about 1 cm/s. From this it will be appreciated that slow drying offilter elements as they are continuously extruded, in microwave ovensinto which they are extruded, taking up to 30 minutes or more, wouldresult in impractically long drying ovens. The method of the invention,however, allows much shorter drying times and ovens to be used.Furthermore, it is to be noted that, when 100% power was applied for 2.5minutes, followed by 50% power for 2 minutes and then by 100% power for1 minute, cracks and blisters were noted on the external surfaces of theelements. This indicates that a power reduction, followed by a powerincrease, is unsuitable for rapid drying, and suggests that a lowinitial power followed by a high power may also be unsuitable.Furthermore, blisters were noted in Tests 3 and 4 and confirm theimportance of providing insulation around the elements to reduce heatloss from the external surface of the elements during drying thereof.

It is an advantage of the invention that it allows the body to be driedsufficiently to permit handling and further treatment thereof withoutdeformation or damage thereto as a result of mechanical handling of thebody and/or gravitational forces acting on the body.

What is claimed is:
 1. A method of making a dried porous ceramic bodyhaving an external surface provided with pores in communication withinterconnected channels in its interior, the method comprising the stepsof:consolidating, by extrusion thereof, a particulate ceramic materialin the form of a paste, to form a porous green ceramic body; drying thebody by subjecting, simultaneously with the extrusion thereof, the bodyto heat energy input by irradiating it with microwave radiation at afrequency of 0.3-10 GHz for an initial period to cause the body toundergo an average rate of temperature increase over the initial periodof at least 15° C./minute; and continuing to subject the body, as it isextruded, to said microwave radiation at a frequency of 0.3-10 GHz for asubsequent period, the average rate of temperature increase of the bodyduring the subsequent period being at most 1° C./minute.
 2. The methodof claim 1,which comprises the step, as the body is extruded, ofresisting heat energy loss from the external surface of the body.
 3. Themethod of claim 2, wherein there is a ratio between the duration of theinitial period and the duration of the subsequent period of 1:10-10:1.4. The method of claim 3, wherein said resisting heat energy loss fromthe external surface of the body comprises heating the external surfaceof the body by means of radiant heat, radiated on to the externalsurface of the body from at least one radiant heat source.
 5. The methodof claim 3, which further comprises heating the environment surroundingthe body, as it is extruded, to cause the temperature of the environmentto increase progressively as the temperature of the body increases, saidenvironment being defined by a cavity having a wall or walls directed atthe porous body and by the atmosphere in the cavity around the body, thebody being extruded into the cavity.
 6. The method of claim 1, whereinthe body is formed with a moisture content, before drying, of 14-16% bymass, the drying acting to reduce the moisture content thereof by 2-5%by mass, to a value of 9-14% by mass.
 7. The method of claim 1, whereinthe initial period is measured, for any part of the body, from themoment of extrusion of that part, and the extrusion being into a cavityhaving a wall or walls spaced at most 100 mm from the body, thetemperature of the external surface of the body and the wall surfacetemperature of the cavity increasing simultaneously and progressively inthe direction of extrusion of the body during the initial period.
 8. Themethod of claim 5, which includes causing or allowing moisture expelledas vapour from the porous body by the drying to issue from the cavity,so that the pressure in the cavity remains substantially constant duringthe drying.
 9. The method of claim 1, wherein said microwave radiationhas a frequency of 1-10 GHz, the initial period having a duration of0.2-20 min and the microwave radiation being delivered to the body at apower of 2-4 kW during the initial period and at a power of 0.5-2 kWduring the subsequent period.
 10. A method of drying a porous bodyhaving an external surface provided with pores in communication withinterconnected channels in its interior containing a liquid, the methodcomprising the steps of:drying the body by subjecting it to heat energyinput by irradiating it with microwave radiation at a frequency of0.3-10 GHz for an initial period to cause the body to undergo an averagerate of temperature increase over the initial period of at least 15°C./minute; and continuing to subject the body to said microwaveradiation at a frequency of 0.3-10 GHz for a subsequent period, theaverage rate of temperature increase of the body during the subsequentperiod being at most 1° C./minute.